(哈希表) $O(n+m)$

用一个哈希表记下访问过的指针

C++ 代码

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *findFirstCommonNode(ListNode *headA, ListNode *headB) {
        map<ListNode*,int> record;
        int cnt = 0;
        ListNode* h = headA; 
        while(h!=NULL){
            record[h] = 1;
            h = h->next;
        }
        h = headB;
        while(h!=NULL){
            if (record[h]) return h;
            h = h->next;
        }
        return NULL;
    }
};